The purpose of this aperture is to shape the light entering the condenser to fully illuminate the view field of the objective in use.
For proper illumination, the aperture must be adjusted whenever the objective is changed.
The condenser aperture is not meant to control the intensity of the light. It serves to enhance contrast and depth of field.
There is a very simple and quick way to adjust the condenser aperture for proper illumination.
Focus the image. Then close the aperture until the corners of the image just start to darken, then back off until the corners are clear.
Closing the aperture too far results in a loss of depth of field.
To demonstrate the contrast and depth of field effect, a thin section slide of Gneiss was photographed using a 20x objective.
The first photo was taken with the condenser aperture fully open
The second photo at the same location was taken with the aperture set to NA .25
The software alternative to the correct aperture setting is image stacking.
The third photo shows the result of using the CombineZP macro Soft Stack on five successively focused images.
So why not set the correct apeture plus do a series of stacked images?
The fourth photo shows the result of correct aperture setting and four consecutively focused images with the CombineZP macro Pyramid Maximum Contrast.
It works, but looks over sharpened.
